Geoffrey of Monmouth records the story in his Historia Regum Britanniae. He describes how Aurelius Ambrosius returned from his exile in Brittany and burnt Vortigern in his tower. Rallying the Britons, he triumphed over the Saxon invaders and executed their leader, Hengist. Following the victory he decided to set up a great memorial to his triumph at a place called Mount Ambrius. When his carpenters and masons were unable to come up with a suitable awe-inspiring design, Ambrosius commissioned Merlin to create one.

Merlin’s solution was to import a stone circle called the Giant's Round from its home atop Mount Killaraus. Giants from Africa had originally brought the stones to Killaraus. They would pour water over the stones and use it to fill baths in which they cured their sick. Uther Pendragon went with 15,000 men to retrieve the stones but they proved too heavy to move. Laughing, Merlin transported them to Salisbury Plain using his magic and set them up just as they were. According to Geoffrey, they have stood there ever since as Stonehenge.